Fuel cell systems have been proposed as power sources for a varying number of applications, including mobile, vehicular applications. Generally, a fuel cell system includes a fuel cell stack that uses hydrogen to produce an electrical current for powering an external device. The hydrogen may be supplied to the fuel cell stack directly from a hydrogen source. However, because of complications associated with storage of a pure hydrogen source within a vehicle, it is more practical that the hydrogen is provided through the reformation of a hydrocarbon fuel. To this end, an auto-thermal reformer is implemented for reforming the hydrocarbon fuel to produce a reformate stream having a hydrogen component.
Traditionally, vaporized fuel, water (as steam) and air are provided as a mixture from a mixing unit to the auto-thermal reformer. The fuel and water are converted from liquid to vapor form through a vaporization process performed within either independent or combined vaporizers. Typically, a combustor is provided for producing hot combustion gases to sufficiently heat the vaporizer(s), enabling the vaporizer(s) to vaporize the liquid fuel and water channeled therethrough. The vaporized fuel and water (now steam) are channeled into a mixing unit for mixture with air, and further channeled into the auto-thermal reformer for reformation.
Such traditional vaporization systems include significant disadvantages. For example, because the vaporizing energy is provided by an external component (i.e. combustor), modulation of the fuel flow, as a function of system demand, is limited. In other words, the turndown dynamics of traditional systems is limited, whereby the variation of mass flux through the vaporizer and mixing units is unable to achieve the required turndown ratio in response to rapid total mass flux through the auto-thermal reformer (as a function of power demand). Another disadvantage of such systems is that soot tends to form within the mixing unit, thereby inhibiting the proper operation of the system. Further, the operational parameters (i.e. temperatures greater than 500° C.) of traditional vaporization systems may result in auto-ignition of the mixture, prior to entering the auto-thermal reformer. Ignited mixtures reaching the auto-thermal reformer can damage the catalyst therewithin and results in inefficient, undesirable reformation reactions. Presently, to avoid auto-ignition a complex control strategy for volume flows into the auto-thermal reformer is employed. This increases the cost and complexity of the fuel cell system.
It is known in the art to implement a cold-flame vaporizer in a system where fuel is oxidized to enable partial conversion of the fuel. However, such applications rely on significant pre-heating of input streams, coupled with strict control of residence time in a reaction chamber, flow control of inerts or transfer of heat to the reaction chamber heat sink to control fuel conversion and prevent auto-ignition. This type of system is disclosed in German Publication No. DE 198 60 308. These strict control requirements pose disadvantages, particularly in cases where fuel conversion forms a part of a larger system, where cost, complexity, operable flexibility and system integration issues are important.
Therefore, it is desirable in the industry to provide an improved vaporizer and vaporization process that alleviates the disadvantages of traditional vaporizers and processes.